1. Field of the Invention
The present invention relates to a method of manufacturing a toner, and the toner manufactured by the method.
2. Description of the Background
With the number of output images per unit of time increasing year by year, there is market demand for ever-faster image forming apparatuses. However, to satisfy this demand for higher-speed performance, severe technical hurdles must first be cleared, as described below.
At present, electrophotography is widely used in the field of on-demand digital printing. In particular, the fixing temperature range for fixing an unfixed toner image on a recording medium is required to be fairly wide to prevent occurrence of offset without at the same time adversely affecting gloss.
However, the thermal energy that can be provided to the toner per unit of time at the fixing device is limited and is difficult to increase. On the contrary, of late the thermal energy used tends to be reduced because of concern for the environment. Therefore, the toner tends not to be heated sufficiently, which leads to insufficient melting of the toner present on or near the surface of the recording medium. The toner that is not melted sufficiently during fixing has an insufficient viscosity and thus is severed at the toner layer on the recording medium. Therefore, one part of the toner remains on the recording medium and the other on the fixing roller.
Alternatively, such insufficient melting results in a weak attachment between the recording medium and the toner, in which case all the toner on the recording medium attaches, i.e., offsets, to the fixing roller.
The toner that is caused to adhere to the fixing roller is likely to be fixed at an unwanted portion, i.e., non-image portion, on the following transferred recording media, which causes production of abnormal images with ghost images. That is, a phenomenon known in the art as offset, referred to as cold offset in this case, occurs in the absence of sufficient heat.
In addition, even when the cold offset phenomenon does not occur, insufficient melting of the toner during fixing causes degradation of the quality of the output image such that the gloss decreases markedly.
To solve these problems pertaining to insufficient melting, intensive research and development have been conducted on how to manufacture a toner containing resins, releasing agents, fixing property improver, etc., having a low melting point in order to improve the low temperature fixing property.
For example, Japanese patent application publication no. 2007-72333 (JP-2007-72333-A) describes a teaching of regulating the difference in the endothermic peak of a toner between prior to the thermal treatment to the toner and after preservation thereof for 72 hours at 45° C. In addition, for example, JP-2007-206097-A describes a method of improving the low temperature fixing property, high temperature storage, and offset resistance of a toner by regulating the ratio of the Fourier transform infrared (FTIR) spectrum of the crystalline polyester contained in the toner as a raw material between prior to preservation at a high temperature and after preservation for 12 hours at 45° C.
Similarly, Japanese patent no. 3478963 describes a method of improving color reproduction property at high density, offset resistance, and characteristics of charge rising by regulating the amount of charge of a toner (that is, ratio Z of charge rising obtained by Q20/Q600×100, where Q20 represents the amount of charge obtained after a toner having a density of 5% is stirred and mixed with a carrier for 20 seconds and Q600 represents the amount of charge obtained after the toner is stirred and mixed with the carrier for 10 minutes) in addition to regulation of the dispersion diameter of the coloring agent in the binder resin contained in the toner and the dispersion diameter of the releasing agent contained therein.
However, a toner that has excellent low-temperature fixing property generally tends to deteriorate when stored in a high-temperature environment, resulting in solidification of the toner. That is, there is a trade-off between the low temperature fixing property and the high temperature storage.
Therefore, in view of the demand for faster image forming apparatuses, it is desirable to have a toner that can be fixed at low thermal energy. However, such a toner is inferior with regard to storage under high-temperature conditions, so that handling of the toner in a high-temperature environment becomes problematic. Conversely, a toner exhibiting excellent high-temperature storage is difficult to have a good low temperature fixing property, which leads to occurrence of the cold offset phenomenon, etc., thereby causing image quality problems.
One conceivable solution to the above-described conundrum is pulverized toner. In the manufacture of pulverized toner through processes of melting, mixing and kneading, a method is widely used that prevents attachment and offset of the toner to a fixing roller by heating, mixing and kneading two or more kinds of resins having different molecular weights or rheology to impart a good fixing property at low temperatures by melting the resins having a small molecular weight, and a good fixing property at high temperatures by melting the resin having a large molecular weight or a high elasticity/viscosity, thereby providing a wide temperature range for fixing without causing attachment and offset of the toner to the fixing roller at a low temperature and a high temperature (refer to Japanese patent no. 3044595, etc.).
However, when at least two kinds of resins having an extremely different molecular weight or rheology are melted, mixed and kneaded to obtain a wide fixing temperature range, a shear force is not provided to the resins during mixing and kneading according to the difference among the viscosities of the resins, resulting in non-uniform dispersion of the toner. In this case, the portion having a high viscosity and the portion having a low viscosity form a sea-and-island type of structure. In addition, dispersal of the pigment, the releasing agent, or the charge control agent deteriorates sharply, thereby degrading the performance of the machine in terms of fixing, image density evenness, fogging, and particularly the quality of images (color saturation or chromaticness).
This problem is markedly noticeable and difficult in the case of a high-speed machine for which outputting quality images is necessary even as while the thermal energy provided per unit of time is reduced. The decrease in saturation described above results in production of images with serious problems. Therefore, this is one of the urgent problems to be solved for pulverized toners.
On the other hand, color image forming apparatuses are common which include no oil supply unit for the fixing device and use a toner containing a releasing agent in place of supplying oil.
However, it is difficult to manufacture a releasing agent having as small a particle diameter as that of coloring agent, and moreover, uniform addition and dispersion is also difficult to achieve. The releasing agent that is unevenly dispersed and exposed to the surface of the toner particles has a relatively low melting point, and is highly crystalline and brittle, which has an adverse impact on durability, storage, and anti-spent property.
As methods of improving the dispersion property of such a releasing agent, for example, JP-2004-295046-A describes using a master batch thereof prepared by preliminarily melting, mixing and kneading the releasing agent and a resin. However, since the master batch uses a resin having a small molecular weight, the shear force is weak and the releasing agent easily bleeds out during melting, mixing and kneading. Therefore, the releasing agent is not sufficiently dispersed.
For these reasons, the present inventors recognize that a need exists for a method of manufacturing a toner having a good combination of a high temperature offset resistance, gloss, and low temperature fixing and excellent in storage, fixing property, color reproduction property, and image quality, and the toner provided by this method.